Instantiate EdgeFactory with a snclMapper

Allow EdgeFactory to change which mappings will be used from common channels
(i.e., X, Y, Z, etc), to SEED SNCLs (station, network, channel, location)
codes. For now we default to "legacy", for backward compatibility, but we
can specify snclMapper="geomag" to use the modern SNCLs used by the USGS
Geomagnetism Program.